GB1559450A

GB1559450A - Method and apparatus for measuring oxygen content of gaseos mixture

Info

Publication number: GB1559450A
Application number: GB41218/77A
Authority: GB
Original assignee: Charbonnages de France CDF
Current assignee: Charbonnages de France CDF
Priority date: 1976-10-08
Filing date: 1977-10-04
Publication date: 1980-01-16
Also published as: ZA775992B; FR2367285B1; FR2367285A1; DE2745034C2; DE2745034A1; ES463041A1; BE859421A

Description

(54) METHOD AND APPARATS FOR MEASURING OXYGEN CONTENT OF GASEOUS MIXTURE (71) We, CHARBONNAGES DE FRANCE, a French Body Corporate, of 9 Avenue Percier, 75008 Paris, (Seine) France, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of measuring the oxygen content of a gaseous mixture, and to an apparatus for carrying out this method.

Reliable methods for measuring the oxygen content of a gaseous mixture which are used at the present time are mainly those which make use of the measurement of paramagnetism or of polarographic measurements. Apparatuses which apply to these methods are difficult to use and are very expensive.

In addition, because of their constitution these apparatuses are completely unsuitable for use for measuring oxygen in confined spaces, such as cellas, ducts, and underground mine workings, or other such spaces.

It has in addition been proposed to determine the oxygen in a gaseous mixture by diffusing it through the porous wall of sintered material of a chamber in which a methanol vapour tension prevails, followed by burning the methanol vapour in the gas diffusing into the chamber, the combustion of the methanol in the oxygen of the diffused gas being effected by a catalytic heating element whose temperature is measured at the hot junction of a thermocouple.

One difficulty with an apparatus of this kind is the obtaining of a saturating content of methanol vapour, since the latter depends on the temperature and pressure and under low temperature conditions may be insufficient to obtain a sufficient concentration of vapour to achieve the expected rate of combustion for the temperature of the detector element to rise significantly in dependence on the oxygen content alone. A method and an apparatus of this kind are described in a document entitled"Informa tion No. T1"issued by the Safety in Mines Research Establishment of Sheffield (Eng- land) (Crown Copyright 1971).

The method of the present invention is performed using an apparatus available commercially and generally known as an explosimeter or fire-damp detector (metha- nometer) which apparatus has been suitably modified. In such known apparatus, described briefly above a catalytic determining element, generally mounted in a measuring bridge which is normally in equilibrium, throws the bridge out of balance when a combustible gas is burned in the atmosphere being monitored.

Accordingly, the present invention provides a process for the measurement of the oxygen content of a gaseous mixture, such as an atmosphere, which process comprises performing the combustion of a combustible gas or vapour together with a sample of the gaseous mixture in a measuring chamber, the combustion being performed with a heating element, which forms part of a measuring circuit which generates a measurement signal which varies as a function of the temperature attained by the heating element, the flow of the combustible gas and of the gaseous mixture into the measuring chamber being such that, despite combustion occurring on contact with the combustion activating element, the concentration attains a value sufficient to give the maximum possible value of the measurement signal, and this value or another variable directly related to it is measured as representative of the oxygen content of the sample of the gaseous mixture.

In one embodiment of the process of the invention, the activating element is an elec trically heated element and the measuring chamber is completely filled with the gaseous mixture, and the combustible gas is then progressively introduced with a flow such that its concentration increases progressively, at least until the maximum possible value of the measuring signal has been achieved, and thus maximum value is measured.

In a further embodiment of the process, the activating element is an electrically heated element, and while a substantially constant flow of combustible gas is introduced into the measuring chamber near the outlet of the measuring chamber, the measuring chamber is completely filled with the gaseous mixture whose oxygen content is to be determined, then a voltage is applied to the heating element, and the maximum value attained by the measurement signal is recorded as being the maximum possible value.

In a yet further embodiment of the process, a permanent regulated flow q of the combustible gas is established in the measuring chamber and the gaseous mixture is likewise introduced into the said chamber with a flow Q which is controlled in dependence on the measurement signal, in such a manner that a maximum possible value of the measuring signal is obtained, and this value is measured.

It is advantageous for the combustible gas to be an expanded liquefied gas, such as butane or propane, thus making it possible to provide a reservoir thereof in or near the apparatus which is used to perform the process.

One advantage of the present process is that it can be carried out reliably independently of the conditions of pressure and temperature. Furthermore, the present process can be performed using a sturdy apparatus capable of being constructed according to the regulations of electric safety in inflammable atmospheres.

The present invention also provides an apparatus for measuring the oxygen content of a gaseous mixture which apparatus includes an explosimeter device or fire-damp detector (methanometer) having a measuring chamber containing an electrically heated catalytic element and a circuit for measuring the electrical resistance of the said catalytic element, together with means for introducing a gaseous mixture, such as an atmosphere, into the measuring chamber, the explosimeter including, in addition, means for the controlled introduction of a combustible gas directly into the measuring chamber.

It is advantageous for the apparatus to be provided with means of controlling the flow of combustible gas and for the apparatus to contain, or to be connected to, a gas reservoir which is in communication with the measuring chamber by way of a duct and of opening and closing means.

One embodiment of the apparatus of the present invention includes means of control of the opening of the gas reservoir which are associated with means for applying a voltage to the determining element, these means preferably being operable simultaneously.

In a preferred embodiment of the apparatus, control means associated with the operation of the means of introducing the gaseous mixture are operable in association with the operation of the means for applying voltage to the determining element, the operation preferably being in sequence with a time lag of the order of one to five seconds.

In a further modification, the apparatus is provided with an automatic device for the coordinated control of the suction device and of at least the application of a voltage to the determining element.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a conventional explosimeter having a catalytic determining filament which can be adapted to provide an apparatus of the present invention; Figure 2 shows a family of curves representing the values of the signal S measured with the apparatus of Figure 1 as a function of the methane content around the determining filament, for different oxygen contents of the atmosphere; Figure 3 shows a graph giving the maximum possible values of the measuring signal as a function of the oxygen content; and Figures 4, 5, and 6 are diagrammatic views of three different embodiments of an apparatus according to the invention.

In the experiments described below some of the tests are carried out with methane and some with butane or propane. The man skilled in the art will know that there are effectively no differences in behaviour of these gases in a catalytic filament explosimeter of any of the types available on the market, and that the experimental conclusions reached using one of these gases are equally applicable to the other gases mentioned, knowing that, given equality of the explosibility rate in relation to the stoichiometrical ratio, there will be equality of indication of catalytic combustion explo simeters.

Although certain experiments were carried out with methane, propane and butane are preferred as being more practical, since they are easy to store in liquid form in a reservoir.

Catalytic filament explosimeters detect the content of combustible gas by means of the increase in the temperature of a filament immersed in a gaseous atmosphere containing the combustible gas to be detected or determined. The electrically heated filament generally forms part of a measuring bridge which measures, for example with a constant supply voltage, the variations of voltage corresponding to the variations of resistance of the filament resulting from its variations of temperature, the latter in turn being dependent on the variations of the content of combustible gas.

However, it is known that the readings given by a catalytic filament explosimeter are not unequivocal, because in air the signal increases, in relation to 0% of combustible gas, to a maximum value which is generally close to the stoichiometrical content, this maximum being constant for a given gas and for a given oxygen content, the latter generally being that of air, that is to say close to 21%. Beyond this maximum close to the stoichiometrical value the signal decreases again. Reference should be made in particular to Auer-Mitteilungen, Auer Methanometer, group 08 (February 1964).

Nevertheless, for a given gas this maximum value itself varies in dependence on the oxygen content of the air. It is this phenomenon that is applied by the invention.

Figure 1 shows a known explosimeter circuit arrangement, in which a platinum filament D of 80 u of a commercial firedamp detector is supplied by a source U causing a voltage V to appear at the terminals of the filament D connected in series with a resistor C of 1Q and included, together with this resistor, in a bridge containing two other resistors, in this case one being of 12.7Q and the other of 97.1Q.

A signal S appears at the terminals B of the bridge. The Signal S is dependent on the voltage V, which is in turn dependent on the methane content in the atmosphere surrounding the filament D. The voltage V is adjusted to V = 0.760 V when D is placed in pure air. The signal S = 0 when D is in pure air.

Referring to Figure 2 the measured value of the signal S is plotted in mV for different values from 0 to 12% or 15% of the methane content and for concentrations of 4,8,12,15,17,19, and 21% of oxygen, the difference being composed of an inert gas, such as nitrogen or CO2. The values measured for each oxygen content have been joined up to trace the curves giving the variation against the methane content of the measured voltage signal S for each fixed oxygen content. Each of these curves has a maximum which corresponds to the value which has been referred to as the"maximum possible value of the measurement signal".

It is seen that for pure air or air having a low oxygen content this maximum possible value is always achieved before 10% of methane. These"maximum possible values" -Smax-have been plotted in the form of points in Figure 3, where it can be seen that there is a correlation between the oxygen content (on the abscissa) and the"maximum possible value-Smax'' (on the ordinate), which can thus be validly accepted as representative of the oxygen content in the atmosphere around the filament D. Also on Figure 3 there is plotted (as points x) the measurements of the maximum possible values-with butane, and (as points 0) the experimental results with propane.

The curve R can be accpeted as the response curve Smax (in mV) as a function of the O content in %, the nature of the combustible gas selected having little influence on the result of the measurements.

It is pointed out that the correlation previously established is valid whatever the type of circuit arrangement of the determining filament D.

For the purpose of verification of experimentation described above was repeated with methane in an explosimeter chamber containing a determining filament D and a compensating filament replacing the resistor connected in series in the measuring bridge of Figure 1. The"maximum possible value"of the signal is reached for the same oxygen contents as before, but beyond the maximum value the signal is abruptly reversed, instead of decreasing as shown in Figure 2. This well-known phenomenon entails no disadvantage, so that the invention can make it possible to perfect any type of catalytic element explosimeter already known per se.

Nevertheless, for each type of catalytic element and for each type of circuit arrangement of this element it will be necessary in designing the apparatus to adjust the intensity of the heating current of the element so as to achieve combustion of the combustible gas. This is already a very well known problem in the field of explosimetry, where it is necessary to obtain a temperature of the determining filament which is sufficient to effect combustion but is not so high that too much wear of the determining filament occurs. In the present invention it is possible to reduce the wear of the filament by reducing its supply voltage when designing the apparatus. Thus, an explosimeter filament which is normally supplied at 1.3 V takes a supply of only 0.7 V for the determination of oxygen. If the circuit arrangement includes a compensating filament, the latter may be in the same chamber as the determining filament or in a separate fluid-tight chamber, as is known per se, depending on the temperature adopted.

Figure 4 shows diagrammatically a first embodiment of an apparatus according to the invention, which is intended for determining oxygen in air. The apparatus comprises a measuring chamber 11 housing a detector filament D and a compensator filament C which are mounted in a measuring bridge, as is known per se, in accordance with the diagram shown in Figure 1. Their common point is earthed and their terminals 12 and 13 are connected to the terminals 22 and 23 of an explosimeter supply and measuring device 21.

The measuring chamber 11 is provided with an air inlet nozzle 14 having vent apertures 15. On the opposite side it has an air outlet aperture 16 having vent apertures 17. One end of a tube 18 is fitted over the nozzle 16, the other end of the tube being provided with a suction device 19, such as a bulb provided with valves or an electric pump.

The apparatus so far described is the same as a known type of explosimeter. In accordance with the present invention, the following additional features are included.

A butane cylinder 31 is connected to the chamber 11 by way of an opening valve 32, a constriction 33 adjusting the flow, a capillary tube 34, and a capillary tube 35 intro ducing butane into the mterior of the chamber 11.

The opening of the opening valve 32 can be controlled by a push-button 36 which simultaneously closes a contact 24 applying voltage to the measuring bridge.

The operation of the apparatus is as follows : the suction device 19 is operated at least until the chamber 11 has been flushed out and filled with ambient air. After this operation the apparatus can be used as an explosimeter by closing a contact 25 supplying to the filaments a voltage suitable for explosimetry, that is to say the nominal voltage of the aforesaid explosimeter ; this use is however optional. Whether or not it has been used as an explosimeter, the apparatus, whose chamber is now filled with ambient air, can be used for the determination of oxygen. For this purpose the button 36 is pressed, thus applying a suitable voltage to the filaments and to the measuring device 21 and effecting the introduction of butane into the chamber 11. The signal S changes until it reaches a maximum, and then varies in the reverse direction. The maximum value corresponds to the oxygen content of the air which can be shown on a graduated scale, traced on a recorder, or displayed on a digital reading apparatus.

In a modified embodiment the chamber 11 may be supplied with ambient air by means of simple diffusion, as is known per se and as illustrated in Figure 5. The chamber 11 is provided with a compensator filament C in a fluid-tight chamber 9 and is closed by a cap 10 of sintered metal through which ambient air diffuses permanently. The apparatus can be used similarly to that previously described, except that after each measurement it is necessary to wait a sufficient time to enable the chamber 11 to be flushed out by diffusion of ambient air into the chamber.

It is also possible to construct an automatic apparatus with sequential control identical to that of the cycle described as a form of utilisation of the apparatus illustrated in Figure 4, that is to say manual or automatic operation of the suction device 19, termination of suction, introduction of butane and application of voltage, followed by the reading.

It is possible to construct a simplified version of this apparatus in which a slight flow of butane is permanently injected, as illustrated in Figure 6. In this case, however, the butane must be injected into the chamber 11 near the outlet vent apertures 17, the filaments C and D being disposed at a distance from the latter. In this way the cyclical introduction of air effects the flushing-out of the chamber 11 including the entrainment to the outside of the butane which enters through the tube 35, whereas during the measuring phase the butane diffuses into the chamber, in which its content increases progressively until it produces the maximum possible value of the measurement signal.

The cycle of the apparatus is then as follows: the suction device, operated via an operating connection 41 by a cam type clockwork mechanism indicated diagrammatically at 40, draws in air, entraining the butane contained in the chamber, and then the mechanism 40 stops the suction device and applies voltage to the filament heating and measuring device 21. The measurement signal may be stored in a manner known per se.

In all the embodiments described it is possible to provide a long reserve of operation with a very small amount of butane or propane. Assuming a gas flow of 30 cc per minute and an interrogation time of 6 seconds, the life given by a container holding 10 cc of liquid butane or propane, that is to say the size of a cigarette lighter reservoir, will be about 800 measurements, which is amply sufficient for a portable apparatus.

For fixed or semi-fixed apparatuses working with an automatic cycle, with a flow of 30 cc per minute there would be a life of 10 hours with a container holding 75 cc of liquid butane and a life of five months with a simple commercially available 25-litre cylinder.

WHAT WE CLAIM IS:

Claims

1. A process for the measurement of the oxygen content of a gaseous mixture which process comprises performing the combustion of a combustible gas or vapour together with a sample of the gaseous mixture in a measuring chamber, the combustion being performed with a heating element, which forms part of a measuring circuit which generates a measurement signal which varies as a function of the temperature attained by the heating element, the flow of the combustible gas and of the gaseous mixture into the measuring chamber being such that, despite combustion occuring on contact with the combustion activating element, the concentration attains a value sufficient to give the maximum possible value of the measurement signal, and this value or another variable directly related to it is measured as representative of the oxygen content of the sample of the gaseous mixture.

2. A process according to Claim 1, in which the activating element is an electrically heated element and the measuring chamber is completely filled with the gaseous mixture, the combustible gas is then progressively introduced with a flow such that its concentration increases progressively, at least until the maximum possible value of the measurement signal has been reached, and this maximum value is measured.

3. A process according to Claim 1, in which the activating element used is an electrically heated element and while a substantially constant flow of combustible gas is introduced into the measuring chamber near an outlet of the measuring chamber, the measuring chamber is completely filled with the gaseous mixture whose oxygen content is to be determined, then a voltage is applied to the heating element, and the maximum value attained by the measurement signal is recorded as being the maximum possible value.

4. A process according to Claim 1, in which there is established in the measurement chamber a regulated permanent flow of the combustible gas, and the gaseous mixture is likewise introduced into the said chamber with a flow which is controlled by the measurement signal, in such a manner that a maximum possible value of the measurement signal is obtained, and this value is measured.

5. A process according to any one of the preceding claims, in which the combustible gas is an expanded liquefied gas, such as butane or propane.

6. A process for the measurement of the oxygen content of a gaseous mixture substantially as hereinbefore described.

7. A process for the measurement of the oxygen content of a gaseous mixture substantially as herein described with reference to, and as illustrated in Figures 2 to 6 of the accompanying drawings.

8. An apparatus for performing the process of any one of the preceding claims, including an explosimeter device having a measuring chamber containing an electrically heated catalytic element and a circuit for measuring the electrical resistance of the said catalytic element, together with means for introducing gaseous mixture into the measuring chamber, the explosimeter including, in addition, means for the controlled introduction of a combustible gas direct ly into the measuring chamber.

9. An apparatus according to Claim 8, wherein the means for introducing the gaseous mixture into the measuring chamber is a porous wall or a suction device.

10. An apparatus according to Claim 8 or Claim 9 which includes means of regulating the flow of combustible gas.

11. An apparatus according to Claim 8, 9 or 10, which includes, or is connected to, a gas reservoir which is in communication with the measuring chamber via a duct and opening and closing means.

12. An apparatus according to any one of Claims 8 to 11, which includes control means for the opening of the gas reservoir which are assocated with means for applying a voltage to the determining element.

13. An apparatus according to Claim 12, in which the control means for opening the gas reservoir and the means for applying a voltage to the determining element are operable simultaneously.

14. An apparatus according to any one of Claims 8 to 13, which comprises control means for controlling the operation of the means of introducing the gaseous mixture which control means are associated with the means for applying a voltage to the determining element.

15. An apparatus according to Claim 14, in which the control means are operable sequentially with the operation of the means for applying a voltage, the operation being with a time lag of the order of from one to five second.

16. An apparatus according to any one of Claim 8 to 15, which also includes an automatic device for the coordinated control of the suction device and of at least the application of a voltage to the determining element.

17. An apparatus according to Claim 8 substantially as hereinbefore described.

18. An apparatus according to Claim 8 substantially as described herein with reference to Figures 4 to 6 of the accompanying drawings.~~ ~ ~ ~ ~